Direct injection of a gaseous fuel into the combustion chamber of an internal combustion engine is desirable for several reasons. For example, direct injection allows charge stratification, eliminating throttling losses associated with homogeneous charge engines. Additionally, with direct injection late in the compression stroke, a high-compression ratio can be maintained, maintaining efficiency. Further, when the fuel that is directly injected comprises natural gas, propane, or hydrogen, the emissions of NO.sub.X and particulate matter (PM) are significantly reduced. The directly injected gaseous fuel can be ignited with a glow plug, with a spark plug or with pilot diesel fuel. The gaseous fuel needs to be injected at high pressure to overcome the combustion chamber pressure, which is high at the end of the compression stroke. Preferably, the injection pressure is high enough to promote good mixing between the injected fuel and the combustion chamber air.
Direct injection at high pressures presents several challenges. The use of high pressure fuels for direct injection results in high fuel pressures existing within the injection valve or injector. As a result, the injection valve must typically be strongly seated to avoid leakage of the fuel into the combustion chamber between injection events. The valve is "seated" when the valve is closed, for example, in a needle valve, when the sealing surfaces of the valve needle and the valve seat are in fluid-tight contact with each other. Moreover, compared to low-pressure systems, higher forces are needed to open the injection valve. For example, for a needle valve that employs an inwardly opening valve needle, when the needle is in the open position it may be subjected to high forces from the pressurized fuel. Additionally, there is only a small window of time during which the fuel can be injected. For example, at 4500 revolutions per minute (RPM), at full load, all of the fuel is preferably injected in less than 2-3 milliseconds.
Nearly all prior direct fuel injection systems in internal combustion engines have been hydraulically-actuated. These systems rely on a hydraulic fluid to provide the force that is needed to open an injection valve (or valves, when the engine comprises a plurality of combustion chambers). Accordingly, at typical engine operating speeds, hydraulically actuated injectors rely on rapid changes in the hydraulic fluid pressure to open and close the injection valve(s). An injection valve is typically opened by increasing the hydraulic fluid pressure and closed by reducing the hydraulic fluid pressure, such that the opening force applied to the injection valve is reduced, causing the valve to close. However, in the context of a conventional gaseous injector, hydraulic operation presents several drawbacks, including:
the need for additional hydraulic hardware such as a hydraulic pump, valves, and a reservoir for the hydraulic fluid; PA1 the need for a seal to be established between the variable pressure hydraulic fluid and the high pressure gaseous fuel; PA1 increased bulkiness of the injection valve assembly because of the additional hardware requirements; and PA1 delayed response of the system due to time delay of the hydraulic fluid between the electrical valve hardware and the needle that controls gas flow from the injector. PA1 (a) a valve housing comprising: PA1 (b) a valve needle disposed within the valve housing wherein the valve needle is movable between a closed position at which a sealing end of the valve needle contacts a valve seat to fluidly seal the interior chamber from the nozzle orifice, and an open position at which the sealing end of the valve needle is spaced apart from the valve seat whereby the interior chamber is fluidly connected with the nozzle orifice, wherein valve needle lift equals the distance traveled by the sealing end away from the valve seat; PA1 (c) a needle spring associated with the valve needle, wherein the needle spring applies a closing force to the valve needle for biasing the valve needle in the closed position; PA1 (d) an actuator assembly associated with the valve needle, wherein the actuator assembly may be activated to apply an opening force to the valve needle stronger than the closing force, for moving the valve needle to the open position; and PA1 (e) a hydraulic link assembly comprising a hydraulic link having a fluid thickness through which the opening and closing forces are transmitted, wherein the thickness of the hydraulic link is adjustable in response to changes in the dimensional relationship between components of the injection valve to maintain a desired valve needle lift when the actuator assembly is activated. PA1 (a) a valve housing comprising: PA1 (b) a valve needle comprising a cylindrical portion having a sealing end and a piston portion having a pre-load end, the valve needle disposed within the valve housing wherein the valve needle is movable between a closed position at which the sealing end contacts the valve seat to fluidly seal the interior chamber from the nozzle orifice, and an open position at which the sealing end is spaced apart from the valve seat whereby the interior chamber is fluidly connected with the nozzle orifice, wherein valve needle lift equals distance traveled by the sealing end away from the valve seat; PA1 (c) a needle spring associated with the pre-load end of the valve needle, wherein the needle spring is compressed to apply a closing force to the valve needle for biasing the valve needle in the closed position; PA1 (d) an actuator assembly that may be activated to apply an opening force to the valve needle that is stronger than the closing force, for moving the valve needle to the open position, the actuator assembly comprising: PA1 (e) a hydraulic link assembly comprising a sealed hydraulic cylinder disposed about the piston portion of the valve needle, a hydraulic fluid disposed within the hydraulic cylinder, wherein the opening and closing forces applied to the valve needle are transmitted through the thickness of hydraulic fluid whereby the hydraulic fluid acts as a hydraulic link and the thickness is automatically adjustable in response to changes in the dimensional relationship between components of the injection valve to maintain a desired valve needle lift when the actuator assembly is activated. PA1 (a) initiating an injection event by applying a control pulse and accelerating valve opening by raising the value of the control pulse to a spike value that is greater than the value required for the desired lift (the spike value may be up to about an order of magnitude higher than the value required for the desired lift); PA1 (b) reducing the control pulse from the spike value to a value that is needed to provide the desired lift; PA1 (c) reducing the control pulse to a negative value to accelerate valve closing; PA1 (d) increasing the control pulse to a positive value to slow down valve closing to reduce the impact force of the valve needle on the valve seat; and PA1 (e) reducing the control pulse to zero to close the valve.
Moreover, the degree of controllability of the movement of the injection valve is low when the motive force is provided by a pressurized fluid rather than by a directly controllable source. In this respect, it is difficult to do lift control, with some limited capabilities when using double-spring configuration. Therefore, it is desirable to avoid the use of hydraulics to operate gas injectors, particularly for high-speed engines. "Lift" in the context of needle valves is defined herein as the displacement of the valve needle away from its closed/seated position to its open position.
In response to at least some of the drawbacks with hydraulic activation, solenoid actuators have been considered as an alternative for injection valve actuation because of the simplicity and reliability of solenoids. For example, U.S. Pat. No. 5,035,360 (the '360 patent) discloses a directly actuated gas only injector employing a solenoid actuator. However, the application disclosed for the solenoid actuator is an injection valve for a two-stroke engine with an operating speed of 2100 RPM. The '360 patent discloses introducing fuel into the piston cylinder between the times during the engine cycle when the piston is at approximately bottom dead center and 60 degrees after bottom dead center. The '360 patent further discloses injecting fuel at a pressure of about 300 pounds per square inch (psi) (about 2.1 MPa). Accordingly, the fuel is not introduced at pressures as high as the pressure when the piston is at or near top dead center, for example, at pressures of 3,000 psi (21 MPa) and higher. For a constant needle diameter, higher fuel pressures require higher actuating forces and to increase the actuating force of a solenoid it must be made larger. The '360 patent also discloses that "large solenoids of conventional type are inherently slower than small ones". Thus solenoid actuators are not suitable for applications that require a combination of fast response times and high actuating forces, such as, for example, injectors used for high-pressure fuels. Solenoids also do not effectively deliver small amounts of fuel, such as the amount needed at low loads, and particularly at high-speed. For higher speed engines (for example, engines running at 3000 RPM and higher), the minimum opening time of the needle tends to be too long for delivering only a small amount of fuel. The shortest known opening time for solenoid actuated injection valves is about 700 microseconds. Additionally, while solenoid actuators are capable of substantial lift, of the order of 20-thousandths of an inch (500 micron) or more, they do not permit control of the lift. Conventional solenoid technology is only known to be able to offer duration control (how long the valve is open for) and not position control (how much the valve is lifted).
It is also known to use piezoelectric or magnetostrictive actuation (devices which can change their dimensions under the effect of an electric or magnetic field) for directly actuated injection valves. For example, U.S. Pat. No. 5,031,841 (the '841 patent) describes a metering valve using an actuating member, which according to the patent could be a piezoelectric stack or a magnetostrictive actuator. One feature disclosed by the '841 patent is the addition of a diaphragm which has the dual purpose of acting as a spring biasing the valve in its closed position and of providing a seal between the metered fluid and the actuator. The '841 patent discloses an adjusting screw for mechanically setting the position of the actuator within the housing. According to the '841 patent, the valve needle is rigidly connected to the actuator by a pressure pin.
Piezoelectric or magnetostrictive actuation devices have also been used in injection valves to actuate an internal hydraulic control valve. For example, U.S. Pat. No. 5,819,710 (the '710 patent) describes an injection valve within which a servo valve is used. The servo-valve is actuated by an actuating member, which could be a piezoelectric stack or a magnetostrictive material. The actuating member can be controlled to close the servo valve gently to reduce wear and improve service life. According to the '710 patent, the servo valve actuating member can be paired with a insert bolt or stud to compensate for differences in thermal expansion between the actuator and the injector housing. (See column 4, lines 25-48).
U.S. Pat. No. 5,845,852 (the '852 patent) describes another injector that employs a piezoelectric actuator to operate an internal three-way hydraulic control valve to open and close the main injection check valve. The '852 patent describes a piezoelectric actuator acting through the intermediate of a self-locking preload assembly. This self-locking preload assembly has three functions: (i) to compensate for dimensional changes and/or imperfections; (ii) to offset some of the upward force due to fuel pressure; and, (iii) to preload the piezoelectric stack for better performance.
Similarly, U.S. Pat. No. 5,779,149 describes an injector using a piezoelectric actuator acting on a hydraulic control valve through the intermediate of a hydraulic amplifier, which serves to amplify the movement of the actuator. The hydraulic control valve allows the main injection valve to open and close to meter the amount of fuel injected.
A problem with employing a piezoelectric or magnetostrictive actuator to operate a control valve, which in turn controls the flow of a hydraulic fluid to operate an injection valve, is that this arrangement requires the intermediate action of a hydraulic fluid. Any delays caused by the displacement of the hydraulic fluid causes delays in the actuation of the injector. Accordingly, there is a need for an injector that is directly actuated by an actuator without an intermediate active hydraulic operator generating any actuating forces. Another disadvantage of active hydraulically operated systems is that a hydraulic fluid needs to be supplied and drained from a hydraulic cylinder. When diesel fuel is the main fuel used by the engine, the diesel fuel may be used as the hydraulic fluid. However, when a gaseous fuel is the engine's main fuel, a separate hydraulic fluid system would be needed to operate injectors that rely on hydraulic actuation.